1. Field of the Invention
The present invention relates to a rotary encoder and in particular to a rotary encoder for producing two pulse trains with a desired phase difference therebetween. In applications, a rotary encoder of the present invention may be used in a driving system for an image sensor head of a facsimile machine or a recording head of a printer as coupled to a motor, pulley or wire guide in the driving system, thereby detecting the scanning position, scanning speed, scanning direction, etc. of such a head.
2. Description of the Prior Art
A rotary encoder for producing two pulse trains to control the operation of a reader or recorder head in a facsimile or printing machine is well known in the art. A typical example of such a prior art rotary encoder is schematically shown in FIGS. 1 through 3. Such a rotary encoder usually comprises a rotary plate and a stationary slit plate. FIG. 1 shows a rotary plate 1 which is circular is shape and which is provided with a plurality of slits 2 equally spaced apart from each other. The slits 2 are provided to the plate 1 to allow the passage of light therethrough and arranged in the form of a ring. Thus, the outer edges of the slits 2 are located at a radius a from the center 0 of rotation of the plate 1 and their inner edges are all located at a radius b from the center 0.
FIG. 2 shows a stationary slit plate 10 which is to be used in combination with the rotary plate 1. The stationary slit plate 10 is also provided with a first set 11 of slits and a second set 12 of slits, each set having the same number of slits which are structurally the same as the slits 2 of the rotary plate 1. It is to be noted, however, that the relative positional relationship between the first and second slit sets 11 and 12 is such that when the slits of the first set 11 are precisely aligned with the slits 2 of the rotary plate 1, the slits of the second set 12 are misaligned with the slits 2 by a quarter of the slit pitch.
As shown in FIG. 3, the stationary slit plate 10 is disposed above the rotary plate 1 such that the slits of the first and second sets 11 and 12 may be brought into alignment with some of the slits 2 alternately. Disposed above the stationary slit plate 10 are first and second light emitting elements 31 and 32 such as L.E.D's.; whereas, disposed below the rotary plate 1 are first and second light receiving elements 21 and 22 such as photodiodes. Accordingly, the first light receiving element 21 may receive light emitted from the first light emitting element 31 through the slits of the first set 11 and the slits 2. Likewise, the second light receiving element 22 may receive light emitted from the second light emitting element 32 through the slits of the second set 12 and the slits 2. It should be noted that the rotary plate 1 may be rotated clockwise or counter-clockwise with respect to the stationary slit plate 10 as indicated by the solid line and dotted line arrows.
Under the circumstances, when the rotary plate 1 is rotated to cause relative movement between the slits 2 and the slits of the first and second sets 11 and 12, the first and second light receiving elements 21 and 22 supply output signals shown by waveforms E and F in FIG. 4. These waveforms E and F may be converted into pulse trains e and f by a well-known method. It is to be noted that there is a phase difference of 90.degree. between the pulse trains e and f. In FIG. 4, "T" indicates time, and "N" and "R" indicate rotational direction of the rotary plate 1 with "N" indicating normal rotation and "R" indicating reversed rotation. Thus, when the rotary plate 1 is in normal rotation, the pulse train e is 90.degree. ahead of the pulse train f; whereas, when the rotary plate 1 is in reversed rotation, the pulse train f is 90.degree. ahead of the pulse train e. This leads to the fact that, by detecting the phase difference between the pulse trains e and f, the rotational direction of the rotary plate 1 may be determined. Moreover, the rotational speed and the amount of rotation of the rotary plate 1 may be determined by using at least either one of the pulse trains e and f.
When the slit pitch is small and/or the rotational speed of the rotary plate 1 is high, the cycle of the pulse trains e and f is short so that the phase difference between the two pulse trains e and f is difficult to detect. In other words, it is now required to detect pulse edges of the pulse trains e and f at high speed. Such a difficulty mainly stems from such reasons as possible manufacturing errors in the slits; possibly differing photoemitting characteristics between the two light emitting elements 21 and 22; possibly differing photosensitivity characteristics between the two light receiving elements 31 and 32; and possibly differing conversion characteristics in converting the analog signals E and F into pulse trains e and f, respectively.
Because of the above, the phase difference between the two pulse trains e and f cannot be precisely set at 90.degree. and it also fluctuates due to dimensional errors in individual slits. As the actual phase difference deviates more from the intended value of 90.degree., it becomes more difficult to determine the rotational direction of the rotary plate 1 and a probability of erroneous determination increases. In particular, when the slit pitch is small and/or the rotational speed of the rotary plate 1 is high, the amount of deviation from the intended phase difference of 90.degree. becomes more material in the accurate determination of the rotational conditions of the rotary plate 1 because the cycle of the pulse train e, f is short. The phase difference is also sensitive to the distance between the rotary plate 1 and the stationary slit plate 10 as well as to position and orientation of the light emitting 21, 22 and receiving 31, 32 elements. Accordingly, in accordance with the prior art, an elaborate and time-consuming step of precise positioning of elements is required.